Feeding of fat-free diet for an extended period results in essential fatty acid deficiency (EFAD) in many animal species including man especially in infants and weanling animals. Evidence has been obtained that acutely ill neonates would become EFA deficient within 4-7 days when oral feedings are not tolerated. Biochemical deficiency based on decrease in plasma linoleic and arachidonic acids becomes evident within 2-3 weeks in normal young adults fed a fat-free diet. Recently, solutions containing glucose and amino acids or protein hydrolysate along with vitamins and minerals have been given to patients for total parenteral nutrition. As the result of long-term parenteral nutrition devoid of fats, clinical signs of EFAD and the increase in plasma triene (20:3 w9) to tetraene (20:4 w6) ratio have been observed. Since there are many nutritionally depleted ill, hospitalized patients who can take little food by mouth, it is likely that the incidence of EFAD is higher than realized. This is true especially in those on total parenteral nutrition without fat. Thus, this project is designed to study: l) problems related to early detection of EFAD, 2) prostaglandin synthesis both in EFA deficient and repleted patients in efforts to determine if depletion of EFA will decrease prostaglandin synthesis which may occur in the early phase of EFAD, 3) optimal requirement of EFA in correcting deficient changes, 4) the possible changes in red blood cell and platelet functions and metabolism as the result of EFAD, 5) effect of EFAD on wound healing and 6) morphologic changes of some tissues of patients and animals with EFAD. The proposed project emphasizes studies in patients. Some aspects of this project will also be studies in puppies and weanling rats. Key Words: Essential fatty acid deficiency in man. Prostaglandin Synthesis. Red blood cell and Platelet function and metabolism. Parenteral nutrition. Wound healing.